1. Field of the Invention
The present invention relates generally to antennas and more particularly, to antennas which are responsive to different frequencies and polarizations.
2. Description of the Related Art
By definition, polarization refers to the direction and behavior of the electric field vector in an electromagnetic signal which is radiating through free space (i.e., empty space with no electrons, ions or other objects which distort the radiation). In signals with linear polarization, the electric field vectors sinusoidally reverse their direction in a plane which is orthogonal to the radiation path but they do not rotate. If the orientation of the vectors is vertical, the signal is said to have vertical polarization; if the orientation is horizontal, the signal is said to be have horizontal polarization.
In contrast, if the direction of the electric field vectors rotates at some constant angular velocity the signal has elliptical polarization. Signals with elliptical polarization can be effectively generated by combining two linearly polarized signals which are oriented in an orthogonal relationship and which have a predetermined phase difference between their electric field vectors. Circular polarization is a special case of elliptical polarization in which the two linearly polarized signals have electric field vectors of equal magnitude and a phase difference of 90.degree..
Elliptical polarization may be either right-handed or left-handed. In right-handed polarization, the vector direction rotates clockwise as seen from the radiative element which radiated the signal. The vector direction rotates counter-clockwise in left-handed polarization. Antennas which are designed to receive signals which have one of these elliptical polarizations will typically tend to reject signals which have the other polarization (e.g., in an antenna which is designed to receive right-handed polarization, the gain of a signal with left-handed polarization will be significantly reduced from the gain of a signal with right-handed polarization).
When an elliptically polarized signal is reflected from a conductive surface, its rotation is reversed. That is, if a transmitted signal with right-handed polarization strikes a reflecting surface, the reflected signal will have left-handed polarization. The reflected signal will be received with less gain than the transmitted signal by an antenna which is designed to receive right-handed polarization. Consequently, signals with elliptical polarization have an inherent resistance to multipath distortion; this is one reason why satellite communication is typically conducted with circularly-polarized signals.
Various communication systems require the transmission and reception of signals with different frequencies and polarizations. For example, cellular telephone systems have conventionally divided large service areas into smaller cells which each have a terrestrial transmitter. In a particular cell, different hand-held wireless telephones communicate through the cell's transmitter on a terrestrial (cellular) frequency with linear polarization. In a satellite-based system, satellites are combined with ground-based "gateways" such as a telephone exchange or a private dispatcher to facilitate communication between widely-spaced mobile users. To communicate through the gateways, different hand-held wireless telephones communicate on an extra-terrestrial (satellite) frequency with circular polarization.
Therefore, a cellular telephone which is intended for both terrestrial and extra-terrestrial communication preferably responds to a linearly-polarized signal having a first frequency with significant azimuthal gain and responds to a circularly-polarized signal having a second frequency with significant elevational gain.
A conventional antenna structure for such a cellular telephone has two antennas which are connected by a diplexer. Each leg of the diplexer is intended for passing a different one of the frequencies and includes, therefore, a filter network which has a significant insertion loss at the other of the frequencies. Although this structure can respond to the terrestrial and extra-terrestrial signals, its additional filter networks add size and cost to cellular telephones which inherently have limited space and which are directed at a cost-conscious consumer.
Quadrafilar helical antennas (QHA) can also be designed to respond to linearly-polarized and elliptically-polarized signals. An exemplary QHA has four input terminals which must each be fed with different, predetermined phase relationships to obtain the different polarizations. Although this antenna structure can also respond to linearly-polarized and circularly-polarized signals, a diplexer is required to realize the necessary phasing. In addition, QHA gain is typically directed azimuthally which detracts from the usefulness of QHA structures in satellite communications.